The constraints placed on power generation in terms of environmental impact and economics have focused attention on the development of high efficiency, low emission systems. Increasing in the thermal efficiency of a power plant is most effectively achieved by increasing the temperature of the steam driving the power-producing turbine. Currently, typical steam power efficiencies are around 42%, with steam temperatures of 600° C. and pressures of 25-30 MPa. Increasing the operating steam temperature to 625-650° C. will enable thermal efficiencies of around 45% to be achieved. However, the increasing operating temperatures and pressures impose increasingly stringent requirements on the materials of construction.
A well-known material capable of satisfying the requirements noted above is austenitic stainless steel. However, austenitic stainless steel is relatively expensive, and its use in commercial plants is limited for economic reasons. In addition, austenitic stainless steel has a large thermal expansion coefficient and can experience relatively large thermal stresses during transient plant operations, start-up, and shutdown. For these reasons, the use of austenitic stainless steel in plants is problematic. More often, 9 Cr-1 Mo steels, such as ASTM P91 and ASTM P92, among others, are used as an effective-compromise to balance cost and high-temperature demands.
The 9 Cr-1 Mo steels such as ASTM P91 and ASTM P92, among others, generally provide sufficient strength, resistance to corrosion and oxidation, low thermal expansion, and adequate fatigue resistance. The high chromium (Cr) content in these steels results in an oxide film composed of outer layer iron (Fe) oxides and inner layer Cr oxides or Fe—Cr oxides. Generally, Cr in an amount of not smaller than 8.0% is necessary to form a sound oxide film, while an upper limit of approximately 9.5% is established to allow consistent weldability. Molybdenum (Mo) is used as a solid-solution hardening element and a precipitation-hardening element to form highly dispersed carbides and improve the high temperature creep strength of the steels. Mo is limited to approximately 1% or less, because exposure of the 9 Cr-1 Mo steels with Mo at 600-650° C. has been shown to result in the precipitation of Laves-phase, which removes the element from solid solution and reduces solid-solution strengthening. Additionally, these steels have a typical carbon (C) content of approximately 0.1 wt %, which provides sufficient strength while allowing the material to respond well to hot and cold bending, as well as to welding. The stress rupture strengths of these steels are increased by the addition of carbide formers Niobium (Nb) and Vanadium (V). Tungsten (W) is further added to ASTM P92 to allow operations at slightly higher temperatures than P91, but at increased cost. However, in the currently sought temperature environment of 625-650° C., none of the currently used high-temperature steels such as ASTM P91 and ASTM P92, among others, have a satisfactory level of resistance to oxidation and corrosion, and typically the highest service temperature achievable is limited to 625° C.
The resistance to oxidation and corrosion at higher temperatures can be achieved by increasing the content of Cr to improve oxidation resistance, and adding nickel (Ni) to suppress any resulting 6-ferrite, however a high alloy steel with a high content of Cr and Ni significantly increases cost and becomes comparable to an 18-8 austenitic stainless steel from an economic standpoint. Similarly, cobalt (Co) can be utilized to improve the performance of 9 Cr-1Mo steels at higher temperature, but like W and Ni, the addition of Co can be unattractive economically. It would be advantageous to produce a material similar in composition to commonly used high-temperature steels such as ASTM P91 and ASTM P92 that utilizes a relatively inexpensive alloying addition for increased high-temperature performance.
Titanium (Ti) is an economically attractive alloying element and has been investigated for 9 Cr-1 Mo steels. Typically, Ti has been added as a stabilizer preventing sensitization for applications where high strength requirements limit the degree to which C can be reduced. This practice exploits the stronger tendency of Ti over Cr to form carbides, thus permitting the matrix to retain the corrosion inhibiting Cr. However, it is known that Ti can impart brittleness, and the use of Ti as a stabilizer typically emphasizes a Ti content as low as possible, but at a ratio to C or C plus nitrogen (N) on the order often or more. See Grubb, et al, “Micromechanisms of Brittle Fracture in Titanium-stabilized and {acute over (α)}-Embrittled Ferritic Stainless Steels,” Toughness of Ferritic Stainless Steels, American Society of Testing and Materials STP 706 (1980). This combination of requirements tends to necessitate a relatively low carbon level of typically 0.03% or less when Ti stabilization is utilized, which limits application where higher strengths and hardness are required. See U.S. Pat. No. 5,851,316, issued to Yazawa, et al, issued Dec. 22, 1998; U.S. Pat. No. 5,843,370, issued to Koyama, et al, issued Dec. 1, 1998; U.S. Pat. No. 5,051,234, issued to Shinagawa, et al, issued Sep. 24, 1991; U.S. Pat. No. 4,640,722, issued to Gorman, issued Feb. 3, 1987; U.S. Pat. No. 4,461,811, issued to Borneman, et al, issued Jul. 24, 1984; U.S. Pat. No. 4,261,739, issued to Douthett, et al, issued Apr. 14, 1981; U.S. Pat. No. 3,953,201, issued to Wood, et al, issued Apr. 27, 1976. Ti and Nb have also been used in combination for stabilization, but low carbon levels remain a requirement. Additionally, Mo is often treated as an optional or impurity element. See U.S. Pat. No. 4,964,926, issued to Hill, issued Oct. 23, 1990; U.S. Pat. No. 4,834,808, issued to Hill, issued May 30, 1989; U.S. Pat. No. 4,581,066, issued to Maruhashi, et al, issued Apr. 8, 1986.
Ti has also been utilized in 9 Cr-1 Mo steels as a carbide-forming agent which contributes to precipitation strengthening. Precipitation strengthening with Ti requires the dissolution of primary titanium carbides by austenization at high temperature, often greater than 1300° C., in order to dissolve the low-solubility primary titanium carbide as completely as possible. On reheating, fine precipitates of secondary titanium carbide typically less than 30 nm in size distribute throughout the matrix and provide strengthening by acting to impede the movement of dislocations. Dissolution of all or most of the primary titanium carbide during austenization is usually specified, and remaining primary titanium carbides are strictly minimized to avoid degradation of creep properties. Hot working in the austenite temperature range can also be specified to further promote the dissolution of the primary titanium carbides. The latter step, in particular, adds significant processing time and cost to a typical heat treatment that might otherwise consist solely of austenization, cooling, and tempering. See e.g., U.S. Pat. No. 5,310,431, issued to Buck, issued on May 10, 1994; U.S. patent application Ser. No. 11/250,492, submitted by Fujitsuna, et al, published Mar. 16, 2006; U.S. Pat. No. 6,514,359, issued to Kawano, issued Feb. 4, 2003.
It would be advantageous to provide an improved 9Cr-1Mo steel material primarily utilizing an additive alloying element, Ti, that is relatively inexpensive as compared to W, Ni, Co, or other alloying element additions, in order to produce a material comparable in cost to currently used high-temperature 9 Cr-1 Mo materials such as ASTM P91 and ASTM P92, among others. It would be additionally advantageous if the 9Cr-1Mo steel could be fabricated through an austenization, rapid cooling, tempering, and final cooling cycle to avoid costly and time-consuming requirements associated with hot-working in the austenite temperature range. It would be additionally advantageous is the 9 Cr-1 Mo steel provided improved high-temperature creep strength and improved oxidation and corrosion resistance in a temperature environment of 625-650° C. as compared to typical 9 Cr-1 Mo materials such as ASTM P91 and ASTM P92.